In the field of hard copy production, key performance specifications are print quality and throughput, that is, trying to achieve the highest number of copies per unit of time while maintaining print copy quality.
Many inks, dyes, toners, and the like used in the production of hard copy are of a liquid base. This causes a problematical phenomenon known as paper cockle. For example, when the liquid ink is deposited on a wood-based paper, the paper absorbs the ink into the cellulose fibers and causes the fibers to swell. As the fibers swell, they generate localized expansions which in turn cause the paper to warp uncontrollably in these regions. The final printed sheet of paper is unacceptably wrinkled. This problem crosses printing hardware technology boundaries.
In electrophotography, a latent image on a charged surface area of a photoconductor is developed, by application of an electroscopic toner to the area. The developed image is transferred to a hard copy medium, e.g., paper. (For ease of explanation, plain paper will be used as an exemplary print medium hereinafter; however, as will be recognized by a person skilled in the art, the invention described herein is applicable to all forms of hard copy media such as papers, transparencies, envelopes, and the like, and no limitation on the scope of the invention is intended nor should any be implied.) Both wet toner chemicals and dry toner powders are known to be used to develop the image. Heat fusible toner particles are used in liquid developers. The image is then fixed, that is, fused to the paper.
This same electrophotography construct generally applies to printers (HP and Laser Jet are registered trademarks of Hewlett-Packard Company, Palo Alto, Calif.).
In ink-jet hard copy production, text or graphics is produced by scanning a printhead across the paper. The printhead, which in combination with an ink reservoir is sometimes referred to as the ink-jet pen, includes a nozzle plate combined with resistors and drive circuitry. The resistors are used to momentarily boil ink to eject microscopic droplets of ink from the pen onto the paper in a dot matrix configuration to generate the text or graphics. The ink is generally in a liquid form when it first contacts the paper. The carrier fluid evaporates, leaving the colorant dot on the paper. With drop per inch ("DPI") density on the paper approaching 1200 DPI, it is important not only for throughput but also for print quality that the droplets dry in an optimal manner.
Thus, a print quality and throughput design requirement for both toner and ink application in hard copy machines is that a finite time to be allotted for drying so that the print will not be smudged if handled prematurely. The problem is even more pronounced when printing or plotting on plastic-based films and transparencies which have slower rates of absorption than paper materials.
In electrophotography and laser printing, it is known to apply heat to the paper during the fusing process (which is why fresh copies from a photocopier feel warm to the touch). For example, a heated roller (typically having a heating element within such as disclosed in U.S. Pat. Nos. 4,952,781 (Kozaiku) and 4,780,078 (Masui), Japanese Patent No. 62-130S864(A)(Mita)) is generally used to apply pressure to the toner on a sheet of paper pulled through the roller at a rate that causes the toner to fuse properly to the paper.
Another method and apparatus for fusing toner into a print medium during laser printing is shown in U.S. patent application Ser. No. 08/132,598 by Ingram (assigned to the common assignee of the present application) filed on Oct. 6, 1993, incorporated herein by reference. In that invention, a high energy laser beam is synchronized with a low energy beam used to develop the latent image. A follower roller is used, if needed, to sustain the laser induced fusion to fix the developed image. While highly successful, this apparatus is relatively complex and costly. Energy from light other types of light sources have also been used. See e.g., Japanese Patent No. 62-109645 (A)(Abe).
In ink-jet technology, various attempts at solution of the problem have been invented. Medin et al. disclose a print zone heater screen in patent application Ser. No. 07/876,942, filed May 1, 1992, U.S. Pat. No. 5,329,295. Richtsmeier et al. disclose a print heater having variable heat energy for different media in patent application Ser. No. 07/876,986, filed May 1, 1992 and continued at Ser. No. 08/137,388. Russell et al. disclose an airflow system for thermal ink-jet printers in patent application Ser. No. 07/876,939, also filed May 1, 1992, U.S. Pat. No. 5,296,873. Richtsmeier et al. also disclose a heater blower system for color printers in patent application Ser. No. 07/876,924, filed May 1, 1992, U.S. Pat. No. 5,428,384. In U.S. Pat. No. 5,287,123, Medin et al. disclose a preheat roller for pre-print thermal treatment of media.
Another problem in the state of the art is that time is generally wasted while known print fusion heaters are brought up to operating temperature, increasing the time to first page printout.
Due to constant operation when the hard copy machine is in service, a further problem is that fusion heaters generally have to be replaced after a certain number of pages of print, mainly due to heating element failure.
Due to the inherent operating nature, fusion heaters can also be considered a fire hazard.
A related problem is created by the changing thicknesses and coefficients of friction of different types of print media, e.g., plain papers, glossy papers, vellums, envelopes, and plastic transparencies. There is a need in hard copy technology for handling different forms of print media transport through mechanisms such as a fusion heater.
In summary, there is a need in hard copy technology for improvements in quick and efficient methods and apparatus for fusing developed images, such as text or graphics made with wet or dry toner or ink (liquid or hot-melt, dye or pigment-based)--collectively and generically referred to hereinafter as "print dye(s)."